とうとう is an adverb meaning "finally", "at last", or "in the end". It indicates that something has happened after a long period of time, a prolonged process, or continued effort/waiting. It emphasizes the sense that a result – whether positive or negative – was reached as the natural or inevitable conclusion of events.
とうとう can express relief or joy when something long-awaited finally happens.
Unlike ようやく, which is mainly used for positive results, とうとう can also describe unwanted or unfortunate outcomes that arrived after a buildup.
とうとう can also be used with negative verb forms to express that something never happened in the end, despite expectation or hope.